Regulated corona generator

ABSTRACT

An apparatus in which electrical charging of a charge receiving surface is regulated automatically for maintaining the charge accepted thereon at a substantially uniform level as the temperature thereof varies.

United States Patent [191 1 3,805,069 Fisher Apr. 16, 1974 [54] REGULATED CORONA GENERATOR 3,699,335 10/1972 Giaimo, Jr. 250/326 Inventor: Donald H. Fisher, Marion, N.Y.

Xerox Corporation, Stamford, Conn.

Filed: Jan. 18, 1973 Appl. No.: 324,628

Assignee:

US. Cl. 250/326, 317/262 A Int. Cl (103 15/02 Field of Search 250/324, 325, 326;

References Cited UNITED STATES PATENTS 6/1971 Vosteen 317/262 X CONTROLLER 96 J Primary Examiner-William F. Lindquist Attorney, Agent, or Firm--. Iames J. Ralabate; Henry Fleischer [5 7] ABSTRACT An apparatus in which electrical charging of a charge receiving surface is regulated automatically for maintaining the charge accepted thereon at a substantially uniform level as the temperature thereof varies.

6 Claims, 2 Drawing Figures H/GH VOL TAGE SOURCE REGULATED CORONA GENERATOR The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

BACKGROUND OF THE INVENTION This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for charging a charge receiving surface having the charge acceptance dependent upon the temperature thereof.

In process of electrophotographic printing, a charge receiving surface, such as a photoconductive surface, is charged to a substantially uniform potential and, thereafter, selectively discharged by projecting a light image of an original document thereon. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document to be reproduced. The latent image is developed or rendered visible by depositing toner particles thereon. This toner powder image is transferred to a sheet of final support material and subsequently fused thereto.

Generally, the photoconductive surface is charged by a corona generator. For example, U.S. Pat. No. 2,836,725 issued to Vyverberg'in I958 discloses a typical corona generator. The corona generator described therein includes a flat-sided, U-shaped shield having inwardly bent lips and a single coronode wire extending along the longitudinal axis between a pair of opposed, spaced insulating blocks mounted on either end, of the shield. Normally, the shield is maintained at ground potential. However, the shield may be at any desired potential relative to the coronode wire suitable for operation. The coronode wire is energized by a high voltage source, and the photoconductive surface is moved relative to the coronode wire at a uniform rate of speed permitting an electrical charge to be deposited thereon.

In the art of electrophotographic printing, it has been established that high quality copies can best be achieved by charging the photoconductive surface to a substantially uniform level. If the photoconductive surface is not charged to a sufficient potential, the electrostatic latent image recorded thereon will be "relatively weak and the resultingdeposi'tion of toner particles will be correspondingly small; 'Contrawise, if the photoconductive surface is over charged, the converse will occur, i.e. the electrostatic latent image obtained upon exposure will be relatively too strong and the resulting deposition of toner material thereon will be cortive surface is dependent upon the temperature thereof. For example, the charge acceptance of the photoconductive surface may decrease as the temperature thereof increases. Typical decreases in charge acceptance of the photoconductive surface range from about 2.2. volts/F to about 5 volts/F. This is particularly significant when initially starting the machine. As the temperature of the photoconductive surface increases, as is typical from a cold start, the charge acceptance thereof decreases resulting in a reduction of the net effective charge potential thereon.

Accordingly, it is the primary object of the present invention to improve the charging of a photoconductive surface to maintain the charge acceptance thereon substantially constant under varying temperature conditions. I

SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention, there is provided an apparatus for charging electrically a charge receiving surface having the charge acceptance dependent upon the temperature thereof.

In the present instance, the preferred embodiment of the invention includes a corona generator and regulating means. The corona generator is arranged to deposit electrical charge on the charge receiving surface. However, as the temperature of the charge receiving surface varies the charge acceptance thereof changes. To compensate for the variation in charge acceptance of the charge receiving surface, means are provided for regulating automatically the corona generator to maintain v the charge accepted on the receiving surface at a substantially uniform level.

BRIEF DESCRIPTION OF THE DRAWINGS photographic printing machine embodying the features respondingly large. Moreover, if the photoconductive surface is overcharged sufficiently it may be permanently damaged.

' It is, therefore, evident that image contrast is related directly to the potential charge on the photoconductive surface prior to exposure. If the photoconductive surface is not uniformly chargedover its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas thereon and a streaky effect'will be visible on the developed image. Reference heretofore has been made merely to "the necessity for uniformly charging the photoconductive surface. However, not only must a uniform charge be deposited thereon, but the photoconductive surface must be adapted to accept this charge. Frequently the charge acceptance of a photoconducof the present invention therein; and

FIG. 2 is a schematic circuit diagram of the corona generator regulating apparatus. While the present invention will be described in connection with the preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF INVENTION For a general understanding of the disclosed electrophotographic printing machine in which the present invention may be incorporated, continued reference is had to the drawings wherein like reference numerals have been used throughout to designate like elements.

FIG. 1 schematically illustrates the various components of a printing machine for producing multi-color copies from a colored original. Although the charging apparatus of the present invention is particularly well adapted for use in an electrophotographic printing machine, it should become evident from the following description that it is equally well suited for use in a wide variety of temperature sensitive and the. charge acceptance thereof is reduced about 5 volts/F as the temperature of photoconductive 12 increases. Other photoconductive materials have their charge acceptance reduced about 2.2. volts/F as the temperature of photoconductive surface 12 increases. A series of processing stations are disposed about the circumferential periphery of drum such that as drum 10 rotates in the direction of arrow 14 it passes sequentially therethrough. Drum 10 is driven at a predetermined speed relative to the other machine operating mechanism from a common drive motor (not shown). The various machine operations are coordinated with one another to produce the proper sequence of events at the appropriate process ing station.

Drum 10, initially, moves photoconductive surface 12 through charging station A. Charging station A has positioned thereat the corona generator of the present invention, indicated generally as 16. Corona generator 16 extends in a generally transverse direction across photoconductive surface 12. This readily enables corona generator 16 to charge photoconductive-surface l2 to a relatively high, substantially uniform potential. Preferably, corona generator 16 is of the type described in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. As is well known, this type of corona generator comprises a coronode wire connected to a high voltage source and supported in a conductive shield that is arranged in a closely spaced relation to photoconductive surface 12'. The shield generally surrounds the coronode wire except for an opening through which the charge is emitted. Preferably, the shield is arranged to attract surplusemissions from the coronode wire. When the coronode wire is energized, corona is generated along the surface of the wire and ions are caused to be deposited on adjacent photoconductive surface 12. However, the charge acceptance of photoconductive surface 12 varies as the temperature thereof changes. Accordingly, it is desirable to regulate corona generator 16 so as to compensate for the variation in charge acceptance of photoconductive surface 12 as the temperature thereof varies. Pursuant to the present invention, regulating means, indicated generally by the reference numeral 18, control automatically corona generator 16 to compensate for the variation in the charge acceptance of photoconductive surface 12 as the temperature thereof varies. The detailed construction of regulating means 18 and the operative relationship thereof with corona generator 16 will be described hereinafter with reference to FIG. 2. Turning once again to FIG. 1, after photoconductive surface 12 is charged to a substantially uniform potential, drum 10 rotates the charged photoconductive surface 12 to exposure station B.

At exposure station B, charged photoconduive surface 12 is exposed to a color filtered light image of the original document. Exposure station B includes thereat a moving lens system, generally designated by the reference numeral 20, and a color filter mechanism shown generally at 22. A suitable moving lens system is disclosed in U.S. Pat. No. 3,062,108 issued to Mayo in 1962, and a suitable color filter mechanism is described in application Ser. No. 830,282 filed in 1969 now abancloned. As shown in FIG. 1, an original document 24, such as a sheet of paper, book, or the like, is placed face down upon transparent viewing platen 26 so as to be readily illuminated by a lamp assembly, indicated generally by the reference numeral 28. Lamp assembly 28, lens system 20, and filter mechanism 22, are moved in a timed relation with drum 10 to scan successive incremental areas of original document 24 disposed upon platen 26. In this manner, a flowing light image of original document 24 is projected onto photoconductive surface 12. Filter mechanism 22 is adapted to interpose selected color filters into the optical light path. The appropriate color filter operates on the light rays passing through lens 20 to record an electrostatic latent image on photoconductive surface 12 corresponding to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image.

After exposure, drum l0 advances the single color electrostatic latent image recorded on photoconductive surface 12 to development station C. Development station C includes thereat three individual developer units, generally indicated by the reference numerals 30, 32 and 34, respectively. A suitable development station employing a plurality of developer units is disclosed in copending application Ser. No. 255,259 filed in 1972. Preferably, the developer units are all of a type referred to generally as magnetic brush developer units. A typical magnetic brush developer unit utilizes a magnetizable developer mix having carrier granules and toner particles. The developer mix is continually brought through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 12 is developed by bringing the brush of developer mix into contact therewith. Each of the respective developer units contain discretely colored toner parcles corresponding to the complement of the spectral region of the wave length of light transmitted through filter mechanism 22, e.g., a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles thereon. Blue and red latent images are developed with yellow and cyan tone particles, respectively.

Thereafter, drum 10 is rotated to transfer station D where the powder image adhering electrostatically to photoconductive surface 12 is transferred to a sheet of final support material 36. Final support material 36 may be, amongst others, plain paper or a thermoplastic sheet. A bias transfer roll, shown generally at 38, recirculates support material 36 in the direction of arrow 40. Transfer roll 38 is electrically biased to a potential of sufficient magnitude and polarity to attract electrostatically toner particles from the latent image re corded on photoconductive surface 12 to support material 36. A suitable electrically biased transfer roll is described in U.S. Pat. No. 3,612,677 issued to Langdon et al. in 1971. Transfer roll 38 rotates in the direction of arrow 40 in synchronism with drum 10 (in this case at the same angular velocity therewith). Inasmuch as support material 36 is secured releasably thereon for movement in a recirculating path therewith, successive toner powder images may be transferred thereto in superimposed registration with one another.

Support material 36 is advanced from a stack 42 thereof disposed on a tray 44. Feed roll 46 cooperating with retard roll 48 advances and separated successive uppermost sheets from stack 42. The advancing uppermost sheet moves into chute 50 which directs the sheet into the nip of register rolls 52. Thereafter, gripper fingers 54 mounted on transfer roll 38 secure releasably thereon support material 36 for movement in a recirculating path. After a plurality of toner powder images have been transferred to support material 36 (in this case three powder images) gripper fingers 54 release support material 36. Support material 36 is then separated from transfer roll 38 by stripper bar 56 and advanced on endless belt conveyor 58 to fixing station B.

At fixing station E, a fuser, indicated generally at 60, coalesces the transferred powder images to support material 36. One type of suitable fuser is described in U.S. Pat. No. 3,498,592 issued to Moser et al. in 1970.

Upon completion of the fixing process, support material 36 is advanced by endless belt conveyors 62 and 64 to catch tray 66 for subsequent removal therefrom by the machine operator.

Although a preponderance of the toner particles are transferred to support material 36, invariably some residual toner particles remain on photoconductive surface 12 after the transfer of the powder image therefrom to support material 36. These residual toner particles are removed from photoconductive surface 12-as it moves through cleaning station F. Here the residual toner particles are first brought under the influence of a cleaning corona generator (not shown) adapted to neutralize the electrostatic charge remaining on the toner particles. The neutralized toner particles are then mechanically cleaned from photoconductive surface 12 by a rotatably mounted fibrous brush 68. A suitable brush cleaning device is described in US. Pat. No. 3,590,412 issued to Gerbasi' in 1971. Rotatably mounted brush 68 is positioned at cleaning station F and maintained in contact with photoconductive surface 12. In this manner, residual toner particles remaining on photoconductive surface 12 after each transfer operation are readily removed therefrom.

It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine embodying the teachings of the present invention.

Turning now to the specific subject matter of the present invention, FIG. 2 depicts schematically regulating means 18 coupled to corona generator 16 and photoconductive surface 12. The construction of corona generator 16 is exemplary of one practical embodiment .that consists of a conductive shield 70, preferably made of aluminum or stainless steel. Shield 70 is of a generally inverted, U-shaped cross-section. The corona generator includes a coronode wire 72 functioning as a discharge electrode. Preferably coronode wire 72 is made from any suitable non-corrosive material such as stainless steel, platinum, or tungsten having a tungsten oxide coating thereon. The wire has a substantially uniform exterior diameter of approximately 0.0035 inches.

Coronode wire 72 extends longitudinally along the length of shield 70 and is connected at either end thereof to suitable dielectric blocks which are made of insulating material and attached to opposed, spaced ends of shield 70. Regulating means 18 includes generating means 74 arranged to develop an output signal indicative of the charge acceptance of the photoconductive surface. Generating means 74 is responsive to temperature variations in the region of photoconductive surface 12 so that the output signal therefrom substantially corresponds to the charge acceptance of photoconductive surface 12 as the temperature thereof varies. In addition to generating means 74, regulating means 18 includes reference signal producing means, or preferably, an impedence voltage source adapted to produce a suitable reference corresponding to the desired charge acceptance of the photoconductive surface. The output voltage from generating means 74 is compared with the reference voltage from impedance voltage source 76 and the difference or error signal is amplified by a suitable electrical amplifier 78. As

shown in FIG. 2, terminal 82 of generating means 74 is connected to terminal 84 which, in turn, is connected to terminal 86 of resistance element 88. In addition, photoconductive surface 12 and terminal 102 of generating means 74 are both electrically grounded. Resistance element 88 is connected to shield so as to detect the portion of current, or corona discharge attracted to shield 70. In this manner, the voltage developed at terminal 84 simulates the charge accepted on photoconductive surface 12 and includes therein the charge attracted to shield 70.

The reference voltage from impedence voltage source 76 is connected to terminal 80, as is generating means 74. The voltage developed at terminal corresponds to the difference or error between generating means 74 and the reference. Thus, the voltage output from terminal 80 is the error voltage corresponding to the requisite change in coronode charge in order to have photoconductive surface 12 accept the charge desired thereon. The error signal developed at terminal 80 is amplified by a suitable amplifier 78 and the resulting amplified error signal is utilized to excite the energizing means or input controller 90 arranged to regulate high voltage source 92 exciting coronode wire 72. One input lead to high voltage source 92 is connected to input controller 90 while the other input lead thereto is connected to terminal 94. Similarly, input controller 90 has one input lead thereof connected to terminal 96 while the other input is the amplifier error signal generated by amplifier '78. Terminals 94 and 96 are connected to an electrical power input preferably having a voltage ranging from about 98 volts to about 127 volts at a frequency ranging from about 50 hertz to about 60 hertz.

Generating means 74 includes thermally responsive means, or thermistor 98 and detecting means, or resistance element 1110. Thermistor 98 is connected in parallel with resistance element 199. Preferably, thermistor 98 has a temperature sensitivity such that the parallel combination of resistance element 100 and thermistor 98 provide a temperature sensitivity substantially simulating that of photoconductive surface 12. Resistance element 100 is, preferably, a one percent resistor with atemperature sensitivity of 100 parts per million or less.

FIG. 2 depicts regulating means 18 connected to shield 70 so that shield current is fed back to high voltage source 92, as well as being fed to generating means 74. One skilled in the art will appreciate that the present invention is not necessarily so limited and that shield 70 may be grounded in lieu of being interconnected with generating means 74 and high voltage source 92.

By way of example, high voltage source 92, preferably is a constant current source adapted to excite coronode wire 72 at 400 micoramps and about 7,000 volts. In this manner, coronode wire 72 is adapted to substantially uniformly charge photoconductive surface 12 to about 900 volts. The output from coronode wire 72 is regulated to vary as a function of the variation in charge acceptance of photoconductive surface 12. Thus, coronode wire 72 will produce a charge sufficient to maintain photoconductive surface 12, preferably at about 900 volts irrespective of temperature variation and the corresponding effect on the charge acceptance of photoconductive surface 12.

In recapitulation, the regulating apparatus of the present invention is adapted to compensate for the change in charge acceptance of the photoconductive surface as the temperature thereof varies. This is achieved in the present instance by varying the change produced by the coronode wire of a corona generator to maintain the charge on the photoconductive surface substantially constant as the charge acceptance thereof varies due to temperature changes. To this end, a thermistor is connected in parallel with a fixed resistance element producing a voltage output which is compared to a desired reference voltage to an error signal. The error signal is amplified and excites an input controller which, in turn, regulates the voltage generated by a high voltage source connected to the coronode wire. The thermistor connected in parallel with the fixed resistance element is excited by the photoconductor current, and is adapted to have a voltage output corresponding to the charge acceptance of the photoconductive surface.

Thus, it is apparent that there has been provided in accordance with the present invention an apparatus for charging electrically a photoconductive surface to a substantially uniform level independent of the temperature thereof that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, itis evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims. What is claimed is:

1. An apparatus for charging electrically a charge receiving surface having the charge acceptance dependent upon the temperature thereof, including:

a corona generator arranged to deposit electrical charge on the charge receiving surface;

means for detecting the charge being de poisted on the charge receiving surface by said corona generator;

' thermally responsive means for sensing the variation in temperature of the charge receiving surface, said thermally responsive means being coupled to said detecting means for generating an output signal substantially corresponding to the charge acceptance of the charge receiving surface;

means for producing a reference signal corresponding to the desired charge acceptance of the charge receiving surface;

means for comparing the reference signal from said producing means to the output signal from said thermally responsive means for developing an error signal corresponding to the deviation therebetween; and

means, responsive to the error signal from said comparing means, for energizing said corona generator to provide a charge therefrom which maintains the charge on the charge receiving surface at substantially about the desired acceptance level.

2. An apparatus as recited in claim 1, wherein said detecting means includes a substantially thermally insensitive resistance element.

3. An apparatus as recited in claim 2, wherein said thermally responsive means includes a thermistor coupled to said resistance element in a parallel circuit arrangement so that the temperature sensitivity of the parallel circuit arrangement substantially corresponds to the temperature sensitivity of the charge acceptance of the charge receiving surface.

4. An electrophotographic printing machine including:

a photoconductive surface;

a corona generator arranged to deposit electrical charge on said photoconductive surface;

means, coupled to said corona generator and said photoconductive surface, for regulating automatically said corona generator to compensate for the variation in charge acceptance as the temperature of said photoconductive surface varies to maintain the charge accepted thereon at a substantially uniform level;

means for detecting the charge being deposited on said photoconductive surface by said corona generator;

thermally responsive means for sensing the variation in temperature of said photoconductive surface, said thermally responsive means being coupled to said detecting means for generating an output signal substantially corresponding to the charge acceptance of said photoconductive surface;

means for producing a reference signal corresponding to the desired charge acceptance of said photoconductive surface;

means for comparing the reference signal from said producing means to the output signal from said thermally responsive means for developing an error signal corresponding to the deviation therebetween; and

means, responsive to the error signal from said comparing means, for energizing said corona generator to provide a charge therefrom which maintains the charge on said photoconductive surface at substantially about the desired acceptance level.

5. A printing machine as recited in claim 4, wherein said detecting means includes a substantially thermally insensitive resistance element.

6. A printing machine as recited in claim 5 wherein said thermally responsive means includes a thermistor coupled to said resistance element in a parallel circuit arrangement so that the temperature sensitivity of the parallel circuit arrangement substantially corresponds to the temperature sensitivity of the charge acceptance of said photoconductive surface. 

1. An apparatus for charging electrically a charge receiving surface having the charge acceptance dependent upon the temperature thereof, including: a corona generator arranged to deposit electrical charge on the charge receiving surface; means for detecting the charge being depoisted on the charge receiving surface by said corona generator; thermally responsive means for sensing the variation in temperature of the charge receiving surface, said thermally responsive means being coupled to said detecting means for generating an output signal substantially corresponding to the charge acceptance of the charge receiving surface; means for producing a reference signal corresponding to the desired charge acceptance of the charge receiving surface; means for comparing the reference signal from said producing means to the output signal from said thermally responsive means for developing an error signal corresponding to the deviation therebetween; and means, responsive to the error signal from said comparing means, for energizing said corona generator to provide a charge therefrom which maintains the charge on the charge receiving surface at substantially about the desired acceptance level.
 2. An apparatus as recited in claim 1, wherein said detecting means includes a substantially thermally insensitive resistance element.
 3. An apparatus as recited in claim 2, wherein said thermally responsive means includes a thermistor coupled to said resistance element in a parallel circuit arrangement so that the temperature sensitivity of the parallel circuit arrangement substantially corresponds to the temperature sensitivity of the charge acceptance of the charge receiving surface.
 4. An electrophotographic printing machine including: a photoconductive surface; a corona generator arranged to deposit electrical charge on said photoconductive surface; means, coupled to said corona generator and said photoconductive surface, for regulating automatically said corona generator to compensate for the variation in charge acceptance as the temperature of said photoconductive surface varies to maintain the charge accepted thereon at a substantially uniform level; means for detecting the charge being deposited on said photoconductive surface by said corona generator; thermally responsive means for sensing the variation in temperature of said photoconductive surface, said thermally responsive means being coupled to said detecting means for generating an output signal substantially corresponding to the charge acceptance of said photoconductive surface; means for producing a reference signal corresponding to the desired charge acceptance of said photoconductive surface; means for comparing the reference signal from said producing means to the output signal from said thermally responsive means for developing an error signal corresponding to the deviation therebetween; and means, responsive to the error signal from said comparing means, for energizing said corona generator to provide a charge therefrom which maintains the charge on said photoconductive surface at substantially about the desired acceptance level.
 5. A printing machine as recited in claim 4, wherein said detecting means includes a substantially thermally insensitive resistance element.
 6. A printing machine as recited in claim 5 wherein said thermally responsive means includes a thermistor coupled to said resistance element in a parallel circuit arrangement so that the temperature sensitivity of the parallel circuit arrangement substantially corresponds to the temperature sensitivity of the charge acceptance of said photoconductive surface. 